At present, in architectural and entertainment lighting applications more and more solid state lighting based on Light Emitting Diodes (LED) is used. LED's or LED units have several advantages over incandescent lighting, such as higher power to light conversion efficiency, faster and more precise lighting intensity and color control. In order to achieve this precise control of intensity and color from very dim to very bright light output, it is necessary to have accurate control of the forward current flowing through the LED's.
In order to provide said forward current through the LED or LED's, a converter (or a regulator such as a linear regulator) can be used. Examples of such converters are Buck, Boost or Buck-Boost converters. Such converters are also referred to as switch mode power sources. Such power sources enable the provision of a substantially constant current to the LED unit. When such a LED unit comprises LED's of different color, the resulting color provided by the LED unit can be modified by changing the intensity of the different LED's of the unit. This is, in general, done by changing the duty cycles of the different LED's. Operating the LED's at a duty cycle less than 100%, can be achieved by selectively (over time) providing a current to the LED's, i.e. providing the LED's with current pulses rather than with a continuous current. As more and more conventional lighting systems such as halogen lighting or light bulbs are replaced by lighting systems using Light Emitting Diodes, it is important to operate such a lighting system efficiently in order to minimize the power consumption associated with it. In general, a lighting system is applied to operate over a range of illumination (or lighting) conditions (e.g. the brightness of lighting system may be set within a certain range). By merely considering the efficiency of the lighting system at e.g. a nominal operating point rather than over the entire operating range or part of the operating range, the power losses of known lighting systems may be important when operating under certain conditions (e.g. a reduced brightness compared to a nominal brightness).
It is therefore an object of a first aspect of the present invention to improve the efficiency of a lighting system using LED's.
It has been described to drive a plurality of LED's by means of time based modulation techniques, such as pulse width modulation, duty cycle modulation algorithms etc. Thereby, the LED's may be divided in groups, wherein each group of LED's e.g. has its own color of light, each group of LED's being driven by a suitable modulation technique with a certain duty cycle. An example thereof is provided in WO2006107199 A2, wherein LED's or groups of LED's are connected in series, the LED's or groups of LED's each being provided with its own switching device connected in parallel to the group or to each LED. A current source is provided to generate a current through the series connection of LED's or groups of LED's. Closing the parallel switch will bypass the LED or group of LED's so as to switch it off.
At a lower intensity, a change in the intensity by an increase or decrease of the duty cycle becomes relatively larger, the smaller the duty cycle. As an example, assuming a 16 bit duty cycle information, a decrement from FFFF (hexadecimal) to FFFE (hexadecimal) provides percentagewise a small reduction, thus enabling a smooth dimming, while a decrement of for example 0009 to 0008 provides percentagewise a large reduction. This effect may be emphasized by a sensitivity of the human eye, which is commonly assumed to have a logarithmic or similar characteristic. Hence, at low intensity levels and low duty cycles, an increment or decrement in duty cycle will result in a relatively more noticeable change than at large duty cycles. Hence, at low intensities, a possibly less smooth change in intensity can be obtained as compared to more large intensities.
Accordingly, an object of a second aspect of the invention is to provide a higher dimming resolution at lower intensities.